This invention relates to a Schottky barrier diode (hereafter referred to as an SBD) and, in particular, to an SBD with a guard ring that has a comparatively high withstand voltage.
SBDs have a low forward voltage drop and use majority carriers so the reverse recovery time is short. These characteristics make the SBD widely applicable as high-speed rectifying elements in power switching applications. Also, as SBDs are formed of a junction of a metal layer and a metal silicide layer on the surface of a semiconductor, an electrical field is formed around the ends of the metal and silicide layers. In order to increase the withstand voltage, an SBD is provided with a guard ring 2, as is shown in FIG. 1. An N.sup.- epitaxial layer 4 is formed on an N.sup.+ semiconductor substrate 3 and on top of this, a Schottky barrier metal layer 1 is formed. Around the edges of layer 1 on the surface region of the epitaxial layer 4, the guard ring 2 is formed, which is a region with a conductivity opposite to that of which the epitaxial layer 4 is formed. An opening is formed in an insulating film 5, which is also formed on the upper surface of the epitaxial layer 4, and a metal electrode 6 is formed on top of the Schottky barrier layer 1. On the underside of the N.sup.+ semiconductor substrate 3 an electrode 7 is formed. In this kind of SBD, the electrical field concentration around the Schottky barrier metal layer 1 is reduced inside the guard ring 2.
In the prior art guard-ring-equipped SBD, the withstand voltage was in the 40 to 50 volt range so, for example, when used in power rectifying applications where the signal frequency is under 500 KHz, no particular problems occurred.
In recent years, however, the demand for SBDs having a withstand voltage of 100-200 V has occurred. If a device having a guard ring such as that shown in FIG. 1 is used without any modifications in such high-power applications, the reverse recovery time is extremely long and the particular features of the SBD are lost. Namely, when the specific resistance of the N.sup.- epitaxial layer 4 is increased in order to increase the withstand voltage, the forward voltage drop in the layer 4 also increases. This increase results both in the junction of the P.sup.+ guard ring 2 and the N.sup.- epitaxial layer 4 becoming forward biased and in a large number of minority carriers flowing from the P.sup.+ guard ring 2 to the N.sup.- layer 4. This influx of minority carriers in turn greatly increases the reverse recovery time of the SBD.
For example, the SBD shown in FIG. 1 has a withstand voltage of 40 V and a reverse recovery time of about 50 ns. If the specific resistance of the N.sup.- layer 4 is increased to give this particular SBD a withstand voltage of 200 V, the reverse recovery time would become 300 ns.
If, for example, a P.sup.+ guard ring was not provided, the reverse recovery time would decrease but, in such a case, an electrical field concentration would be produced around the Schottky barrier metal layer and stable withstand elements could not be obtained.